This invention addresses the problem of sealing semiconductor devices in hermetic ceramic packages with a low temperature sealing glass. Concurrently this invention addresses the problem of bonding (die attach) to a ceramic surface certain types of temperature-sensitive semiconductor devices at the lowest possible temperature. Since the onset of integrated circuits fabricated on silicon single crystal wafers around 1964, very fast semiconductor devices have been designed by a process known as bipolar technology which relies on deep diffusion silicon structures. These devices being somewhat temperature and surface insensitive were readily alloyed, die attached, and hermetically sealed in alumina ceramic packages at 450.degree.-500.degree. C.
A rapidly growing competing design technology based not on pn junction high current injection but on surface capacitive channel switching called CMOS (complementary metal silicon oxide semiconductor) requires much less power to operate. Since the speed of CMOS designs is increasing so quickly they will soon outstrip almost all competing semiconductor technologies with a concurrent increasing impact in portable, work station and mainframe computers. This increased operational speed in CMOS is critically dependent on submicron scale masking technology.
Very large scale integrated semiconductor devices (VLSI) such as large 300 to 600 mil square CMOS and BiCMOS silicon chips are quite sensitive to the thermal processes required during their last fabrication steps. These include metal contact alloying, die attach and final seal. These are presently performed in the 400.degree. to 450.degree. C. range for a few minutes. The thermal sensitivity of CMOS semiconductor devices arises due to the presence of extremely dense, compact, ultrafine metallization lines reaching a fraction of a micron line widths combined with ultrathin dielectric films reaching the 100 angstrom range thickness. These three-dimensional-surface interconnection patterns are prone to immediate or longer term failure modes such as metal diffusion, alloying and dielectric punchthrough. Industry consensus indicates that these fabrication steps should be made below 400.degree. C. and preferably close to 350.degree. C. to insure greater fabrication yields, throughput and long-term reliability.
Glass sealing and silver/glass die attach processes are critically dependent on the available materials which today are derived from the lead borate oxide glass system. Commercial lead borate glasses used for semiconductor packaging applications typically have glass transition temperatures (Tg) in the region of 325.degree. C. and softening points in the region of 375.degree. C. Present package sealing and silver/glass die attach materials require a processing temperature of 430.degree. to 450.degree. C.
The key factor in potentially achieving lower processing temperatures would be the availability of the right glass material. While some very low temperature metal oxide glasses are known to exist with glass transition temperatures in the 250.degree. to 350.degree. C. level, most are not useful for semiconductor application. The limiting factors are thermal instability (the glass recrystallizes too early), mechanical instability (the glass recrystallizes when ground to a fine powder), poor moisture resistance (many metal oxide glasses dissolve in hot water) and the presence in the glass formulation of alkali metals and halides--components known to affect deleteriously the performance of most semiconductor devices.
To date, serious attempts to design a practical and reliable lower temperature (300.degree.-400.degree. C.) sealing glass have met with formidable technical barriers, the search being hampered by the fact that in the course of new material evolution the design of glasses remains largely an empirical science.
The requirements for a semiconductor ceramic package sealing glass are numerous and demanding. Somehow these must be met with one single chemical formulation preferably produced as a glass melt rapidly quenched to room temperature. The basic material and processing requirements for a commercially practical sealing glass can be listed as:
1. formation of a true solution (homogeneous melt) of the metal oxide mixture; PA1 2. glass formation during rapid cooling of the melt (solidified liquid); PA1 3. low glass viscosity at seal temperature (350.degree. C.); PA1 4. no tendency to crystallize (glass stability) during seal formation and completion; PA1 5. a reasonably low linear thermal expansion (50 to 140.times.10.sup.-7 /.degree.C.); PA1 6. ease of linear thermal expansion adjustment by the addition of a lower expansion coefficient filler; PA1 7. glass chemical stability (insoluble in water, resistant to acids, alkalies and hot water); PA1 8. good wetting and high bonding strength to alumina ceramic surfaces; PA1 9. no presence in the formula of alkali or other fast-migrating ions (electronic applications) or volatile components that create serious health hazards (such as arsenic oxide, thallium oxide, etc.); PA1 10. capacity for producing a strong, tight and hermetic seal to a glass, metal and ceramic surface and capability of surviving several hundred cycles of thermal shocks, liquid to liquid, condition C (MIL-STD-883); and PA1 11. ease of commercial processing. PA1 (a) TeO.sub.2 : 55 to 65% PA1 (b) V.sub.2 O.sub.5 : 30 to 40% PA1 (c) Nb205, ZnO and/or ZrO.sub.2 : 1 to 10%.
The present invention concerns glasses derived from the tellurium vanadium oxide binary by the addition of niobium pentoxide, zinc oxide, zirconium oxide and tantalum oxide. Other oxide additives compatible with tellurium vanadium oxide glasses are bismuth oxide and cupric oxide.
One prior U.S. Pat. No. 4,945,071 claims a four component Ag.sub.2 O/V.sub.2 O.sub.5 /TeO.sub.2 /PbO glass. The presently claimed glasses are distinct from this patent in that the present glass system does not contain silver oxide (Ag.sub.2 O). Silver oxide may make the glass unstable, particularly in the presence of fillers. The patent also does not describe the use of niobium oxide, zirconium oxide and/or tantalum oxide as components in glass. These oxides are included in preferred embodiments of the present glass.
U.S. Pat. No. 4,743,302 describes lead vanadium glasses that may contain up to 5% TeO.sub.2. The present glasses are chemically distinct from the glasses of this patent as regards higher content of TeO.sub.2 and lower content of PbO.
U.S. Pat. No. 5,013,697 describes lead vanadium glasses that may contain up to 15% TeO.sub.2. The glasses of the present invention are distinct from the glasses of this patent as regards TeO.sub.2 content.
It is an object of this invention to present a series of stable tellurium vanadium glasses, with or without compatible refractory fillers, suitable for sealing ceramic semiconductor packages at around 350.degree. C. and suitable as well for use in silver/glass die attach systems in the 300.degree. to 400.degree. C. range.